My buddies quads all do it Make sure your caps are tight and check the location of over flow tube. But It will still pressure up especially if you have mods for some reason. Ive seen it on my buddies. He ordered a after market radiator and it was larger and it stopped it.
yeah i found the tube but it was located by the handle bars near the steering stem.. i just got the bike its NUTS compare to my old z 400 lol but anyways i picked it up from my buddy he bought it brand new int 2005.. he put jet kit, high compression piston wisco , web cam kit it comes with came and cam sproket, springs you name it, full Motion pipe(LOUD) and KN air flow kit he never rode the thing! so i bought it for 2500 i think thats a good deal but yeah it pulls hard but i think that high compression piston probbly gonna give me head gasket problems i hope not :X thats what i tought at first i need a head gasket but anyways you guys think thats a good set up? the only bad thing is that it needs a tail light and back bumper because when he first got it it fliped him back thats why i think he never rode it much after that... other than that its pretty sweet i will post a picture